5 week old chick falling over...please send advice

This chick is 5 weeks old. Healthy and plump.
I found it yesterday on it's back, toes curled.
I'm giving vitamins in food and water.
Sometimes it'll get up and eat, and then fall over again.
Wings hang down.
Please someone tell me what this is. I'm very worried about the rest of the flock. I've never had any outbreak of Mareks.
Should I put it down?
Thanks, for ANY advice.